The invention relates to electrolytes for use in electrolytic capacitors.
With the development by Ruben (U.S. Pat. No. 1,710,073) in the mid 1920""s of largely non-aqueous xe2x80x9cworkingxe2x80x9d or xe2x80x9cfillxe2x80x9d electrolytes containing glycerine and borax (sodium tetraborate decahydrate), the working voltage of aluminum electrolytic capacitors was extended to 200+ volts. Ruben""s electrolytes also made possible the modern wound foil and paper separator construction in which the electrolyte is absorbed into the porous separator paper. The use of ammonia/glycerol borate is described on page 72 of the volume, xe2x80x9cThe Electrolytic Capacitorxe2x80x9d Alexander M. Georgiev, Technical Books Division, Murray Hill Books, Inc., New York, 1945.
By the late 1920""s, Ruben developed a series of fill electrolytes based upon ethylene glycol, boric acid, and ammonia solutions (U.S. Pat. No. 1,891,207). These so called xe2x80x9cglycol-boratexe2x80x9d fill electrolytes were found to be capable of satisfactory performance at operating voltages up to about 600 volts. In order to operate above about 450 volts, the ethylene glycol/glycerol and boric acid must be fully esterified and the water removed, and the maximum operating temperature of the capacitor limited to 65 C. or less.
In the 1930""s, Lilienfeld patented a series of fill electrolytes (U.S. Pat. Nos. 2,013,564 and 1,986,779) based upon the condensation products of one or more polyethylene glycols with one or more polyfunctional acids to which finely powdered conductive solids, such as Lamp black, copper powder, or aluminum powder, and a small amount of an ionizable alkali metal salt were added. A typical composition described in these patents is a mixture of the polyester formed from triethylene glycol and boric acid with powdered aluminum (xe2x80x9caluminum blackxe2x80x9d) and a very small amount of borax.
The electrolytes of Lilienfeld, described above, have several advantages over earlier electrolytes. The polyesters formed between polyethylene glycols, such as triethylene glycol, and boric acid may be used to anodize to over 1,500 volts, which is far higher than the maximum voltage attainable with the ethylene glycol/boric acid polyester. The electrolytes of Lilienfeld are thick pastes in the normal operating range of high voltage capacitors and capacitors fabricated with them do not require separator papers or may employ reduced thickness and density of separator papers. Capacitors containing the electrolytes of Lilienfeld are much less susceptible to positive tab corrosion from anodic oxidation products than are capacitors containing ethylene glycol-based electrolytes (tab corrosion by the anodic oxidation products of ethylene glycol is discussed in the paper entitled: xe2x80x9cThe Potential For Positive Tab Corrosion In High Voltage Aluminum Electrolytic Capacitors Caused By Electrolytic Decomposition Productsxe2x80x9d Brian Melody, Proceedings, 13th Capacitor And Resistor Technology Symposium, Costa Mesa, Calif., pages 199-205, 1993).
Unfortunately, the fill electrolytes of Lilienfeld, described above, have some serious disadvantages from the standpoint of capacitor fabrication on a mass production basis. The consistency of the polyethylene glycol polyesters is such that it is very difficult to wet pre-rolled cartridges with them unless very high impregnation temperatures, i.e., approximately 150xc2x0 C., are employed. The conductive solids added to reduce the effective resistivity of these electrolytes tend to separate from suspension when the electrolytes are heated to reduce the viscosity to levels which facilitate traditional vacuum impregnation. The combination of these viscosity and suspension properties is such that wet assembly of the capacitor cartridges or stacks is necessary resulting in much lower manufacturing rates and efficiency than is possible with the electrolytes of Ruben, described above. Additionally, the ionizable salts added to the polyethylene glycol polyesters in order to increase oxide film formation efficiency (see U.S. Pat. No. 1,986,779, page 4, Col 2, lines 40-60) tend to reduce the maximum breakdown voltage of the electrolyte.
Perhaps the largest drawback to the use of Lilienfeld""s electrolytes is the need to employ anode foil which has been anodized so as to produce a duplex anodic film having a relatively thick layer of non-insulting oxide covering a thinner layer of barrier (insulating) oxide in order to prevent shorting due to the conductive particles present in the electrolyte. The thickness of duplex anodic oxides is such as to preclude the use of modem highly etched aluminum anode foils due to the blockage of the etch tunnels by the non-insulating portion of the duplex anodic oxide; only coarsely etched, relatively low capacitance foils lend themselves to use with Lilienfeld""s polyester electrolyte compositions.
The maximum operating voltage of fill electrolytes capable of being use in connection with wound foil and paper cartridges remained at the approximately 600 volt level achieved by Ruben and Lilienfeld until the late 1980""s. Clouse, et al., developed a series of fill electrolytes based upon substituted pyrrolidones and poly-pyrrolidones, some variations of which were found to be capable of operation at voltages in excess of 700 volts (U.S. Pat. No. 5,160,653, Example 8 and column 11, lines 22-36).
More recently, Marshall, et. al., developed a series of electrolytes based upon hydrogen bonded (fumed silica-polar solvent) solutions of certain acrylic monomers which are polymerized in situ (i.e., after absorption by the capacitor cartridges). The resulting electrolytes are claimed to be useful to voltages in excess of 700 volts. Unfortunately, the use of reactive monomeric materials may necessitate the use of glove boxes and other moisture control techniques. The polymerization initiators, such as persulfate compounds, may give rise to corrosive by-products, such as sulfates, which may negatively impact device reliability.
The present invention is directed to a new electrolyte for electrolytic capacitors capable of use at very high voltage, that is 800 or more volts. In one embodiment, the electrolyte of the invention is relatively unaffected by exposure to the atmosphere. Another embodiment provides protection against damage due to hydration of the anodic oxide, and provides good service with aluminum foil of much lower purity than is normally used for the fabrication of electrolytic capacitors.
The present invention is directed to an electrolyte comprising a polyester condensation product of 2-methyl-1,3-propane diol and boric acid; and further comprising dimethyl amino ethoxy ethanol. The amine reduces the resistance of the electrolyte.
In another embodiment, the electrolyte further comprises ortho-phosphoric acid and at least one substituted pyrrolidone or lactone. The at least one pyrrolidone or lactone is preferably at least one of N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-hydroxy ethyl-2-pyrrolidone or 4-butyrolactone, more preferably, N-hydroxy ethyl-2-pyrrolidone. The ortho-phosphoric acid prevents hydration of anodic aluminum oxide in contact with the solution. The pyrrolidone or lactone reduces the resistance of the electrolyte.
In another embodiment, the electrolyte further comprises sodium silicate. The sodium silicate increases the breakdown voltage of the electrolyte.
Although water is generally not added to the electrolyte, minor amounts of water may be present due to the chemicals used.
The invention is further directed to a method of anodizing or healing any faults or cracks in the dielectric oxide covering the anode surfaces of capacitors impregnated with the electrolyte.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present invention as claimed.
It is desirable to produce an electrolyte having a high breakdown voltage, preferably in excess of 800 volts. It is also desirable to produce an electrolyte which may be absorbed into wound or stacked foil and paper capacitor cartridges without the need for xe2x80x9cwetxe2x80x9d assembly of the capacitor cartridges, glove boxes, or other extreme atmospheric exposure control measures. Preferably, the electrolyte is relatively non-corrosive toward the anode foil and tabs. That is, the electrolyte should not contain chlorides, sulfates, or other corrosive anodic species above low ppm levels. Also preferably, the electrolyte should actively contribute to the prevention of hydration of the anodic oxide, if possible, through the inclusion of anionic species known to contribute to anodic oxide passivation.
Co-pending application Ser. No. 09/693,833, now U.S. Pat. No. 6,346,185, describes the preparation and properties of the polyester condensation product of 2-methyl-1,3-propane diol and boric acid. This condensation product is relatively fluid, even at temperatures substantially below the boiling point of water, and is self-ionized to the extent that it may be employed as a high voltage anodizing electrolyte for valve metal anodes. The material has proven so suitable for high voltage anodizing that even relatively impure aluminum anodes have been anodized to voltages of up to 3,000 volts and above in this medium. Unfortunately, this 2-methyl-1,3-propane dioliboric acid polyester exhibits a very high resistivity, in excess of 100,000 ohm-cm, even at the temperature of boiling water, and is therefore unsuitable for use as a fill electrolyte unless it is modified to reduce the resistivity. Furthermore, anionic additives must be added to the formulation in order to achieve greater hydration resistance to the low level of moisture in the electrolyte.
There are many potential cationic materials which might depress the resistivity such as ammonia, alkali metals, and amines. However, relatively few anionic materials adsorb onto and provide hydration resistance to anodic aluminum oxide. Of those materials known to impart hydration resistance to anodic aluminum oxide, the most effective and least toxic materials are those that give rise to the orthophosphate ion in solution. Unfortunately, very few phosphate salts are soluble in organic solvents.
It was discovered that salts formed by the neutralization of ortho-phosphoric acid with dimethyl amino ethoxy ethanol ((CH3)2NCH2CH2OCH2CH2OH), also known as dimethyl ethoxy ethanol amine (DMEEA), are soluble in the polyester condensation product of 2-methyl, 1,3-propane diol and boric acid.
It was further discovered that the resistivity of the electrolyte comprising the polyester condensation product may be reduced substantially by the addition of DMEEA.
Moreover, a small but effective quantity of ortho-phosphoric acid may be added to the electrolyte without precipitation for the purpose of imparting hydration resistance to the anodic aluminum oxide in capacitors containing this electrolyte.
It was then discovered that the resistivity of the polyester condensation product may be reduced further by the addition of one or more substituted pyrrolidones, such as N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-hydroxy ethyl-2-pyrrolidone, etc., and/or lactones, such as 4-butyrolactone or valerolactone. N-hydroxy ethyl-2-pyrrolidone is particularly suitable.
However, although an electrolyte prepared with the polycondensation product, DMEEA, phosphoric acid, and pyrrolidones or lactones, provide low resistance, the breakdown voltage is lower than may be desirable. It was further discovered that adding a small (trace) amount of sodium silicate increases the breakdown voltage of the electrolyte.
The polyester condensation product of 2-methyl-1,3-propane diol and boric acid is described in Ser. No. 09/693,833 which is hereby incorporated by reference in it""s entirety. The polyester condensation product is formed by combining 2-methyl-1,3-propane diol and boric acid and heating to about 130 to about 160xc2x0 C. which drives off the water produced by esterification.
The polyester condensation product is the primary ingredient of the electrolyte. The electrolyte contains a sufficient amount of the dimethyl amino ethoxy ethanol to reduce the resistivity of the electrolyte, preferably to below about 10,000 ohm-cm/100xc2x0 C., preferably about 500 to about 6000 ohm-cm/100xc2x0 C. more preferably about 5000 to about 6000 ohm-cm/100xc2x0 C. Generally, the electrolyte contains about 1 wt % to about 10 wt %, preferably about 2 wt % to about 6 wt %, more preferably about 3.5 wt % to about 4.5 wt %, of the dimethyl amino ethoxy ethanol based on the weight of the polyester condensation product.
Preferably, the electrolyte contains an effective amount of ortho-phosphoric acid or ortho-phosphate to prevent hydration of anodic aluminum oxide in contact with the electrolytic solution. Suitable amounts of ortho-phosphoric acid are about 0.1 wt % to about 1.0 wt %, preferably about 0.5 wt % based on the weight of the polyester condensation product.
The electrolyte further contains about 1 wt % to about 10 wt % of at least one substituted pyrrolidone or lactone, preferably about 6 wt % to about based on the weight of the polyester condensation product to further reduce the resistivity of the electrolyte. The at least one pyrrolidone or lactone is preferably at least one of N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-hydroxy ethyl-2-pyrrolidone or 4-butyrolactone, more preferably, N-hydroxy ethyl-2-pyrrolidone.
In a further preferred embodiment, the electrolyte further comprises sodium silicate in an amount to increase the breakdown voltage. Only trace amounts of sodium silicate are required, and generally not all of the sodium silicate added to the electrolyte dissolves. Generally, since not all of the sodium silicate dissolves, not more than about 1 wt %, preferably about 0.1 wt %, based on the weight of the polyester condensation product is added to the electrolyte.
The invention, in addition to being a working or fill electrolyte, it is also directed to a method of anodizing or otherwise repairing flaws or cracks in an anodic oxide coating on an anodized valve metal substrate by immersing the substrate (usually contained within the body of an assembled electrolytic capacitor) in the electrolyte solution and applying sufficient anodizing voltage to the solution to effect said oxide repairs.
Although not limited to these temperatures, the present method is preferably operated in the temperature range of about 25 to about 85xc2x0 C. The highest voltage anodic oxide films require lower anodizing temperatures of 25-50xc2x0 C., while films formed at higher current densities and to somewhat lower voltages should be produced at temperatures of 50-85xc2x0 C., where the lower viscosity allows a rapid escape of gas bubbles and the lower resistivity gives rise to a more uniform anodic oxide film thickness in a relatively short period of time.
Although any valve metal may be used, the electrolyte and method of the invention are particularly useful as an electrolyte incorporated within an aluminum electrolytic capacitor and useful as a fill electrolyte to convey current between anode and cathode and repair any flaws or cracks in the anodic oxide.